The interaction of platelet factor, a 7,800 dalton protein rleased from platelets during coagulation or in response to blood vessel wall injury, with sulfated glycosaminoglycans of retinal microvessels will be examined. Glycosaminoglycans (GAG) are integral components of the microvascular matrix, where they influence the charge selectivity and permeability of the filtration barrier. Platelet-factor 4 (PF4) carries a net positive charge, has a natural affinity for (carrier) proteoglycan, and binds to heparin; glycosaminoglycans can displace heparin bound to PF4. These considerations suggest that PF4 can bind to glycosaminoglycans of retinal microvessel walls and that, in view of the increased PF4 release and endothelial cell damage in diabetes, this binding contributes to the increased permeability of the retinal microvasculature in diabetes. PF4 will be purified from outdated human platelet concentrates and used to detemine its binding to sulfated glycosaminoglycans isolated from retinal microvessel basement membrane. The latter will be characterized by cellulose acetate electrophoresis and susceptibility to specific enzymatic digestion. Complex formation between PF4 and GAG will be assessed with electrophoresis on cellulose acetate and thin layer chromatography plates, gel filtration chromatography, and 125I(plus/minus)labeled PF4 binding. The latter technique will be adapted to permit measurement of binding, and hence, indirectly, quantification of anionic sites, in microvessel basement membranes purified from the retinas of non-diabetic and diabetic subjects. The results of these studies are expected to provide information on changes in retinal microvascular anionic sites in diabetes and an understanding of the etiologic link between enhanced platelet aggregation and the increased microvascular permeability which occur in this disease.